It’s possible, but I really wouldn’t recommend it.
Before continuing with advice, I am curious ?? why wouldn’t you want to keep the theme responsive? Websites are much more useable and readable on mobile when the design adjusts properly for small screens.
The fastest way I can think of to do what you are asking would be to remove the entire media queries section from the original theme’s style.css file. However, it’s not recommended to edit the original theme files, so you this would be a big enough change in my opinion to simply fork the theme instead of using a child theme. The downside is that you wouldn’t get future theme updates, but it’s a tradeoff I think you’d have to consider for a case like this. There are also other possible ways to make the change you’re asking, but they are very involved and time consuming and I don’t think they would really make sense for this case as it seems like a very rare case to me that someone would want to completely undo a theme’s responsive design.
